I missed your question before Wheelie, but in my opinion a Thommen isn't waterproof. I used to use mine regularly and it usually lived in a wet pocket so I think this is why it stopped working. I think it went to a display of old kit after that!
You might get one cheap off the bay, they seemed to be popular in the states so worth looking on .com too.
that's good to k ow because I have seem some descriptions calling them waterproof